It has been said,
and rightly so, I think, that the icon for the coral reef
aquarium hobby is a clownfish nestled down inside an anemone.
However, if you asked any person at random to pick one animal
that characterizes the marine environment, I suspect they
would answer that it is a sea star or star fish. Sea stars
are the animals everybody brings home from the beach to dry
out and put on a shelf, there to remind them of the ocean
while turning into a pathetic petrified reminder of its former
glory.

Sea stars are members of the group
of animals that scientists call the Class Asteroidea of the
Phylum Echinodermata and they are certainly characteristic
of marine environments. Indeed, they are found nowhere else,
and throughout the ocean bottoms they are often the dominant
animals. Their dominance is not often expressed in vast numbers,
although they may be very abundant, but rather in the fact
they are often keystone predators. Keystone predators are
those predators whose activities control and structure their
biological environment. Indeed, the term keystone predator
was coined for the changes in the intertidal environment caused
by the most common sea star on Pacific Coast of North America,
the ochre star, Pisaster ochraceus (Figure 1).

Figure 1.Pisaster ochraceus,
the Ochre star, this individual is about 5 inches in diameter.
Of course, it isn't ochre colored. There are two common
color varieties, and this is the most attractive, so that
is what I took the picture of.

Anyone who has
been to a rocky seashore is probably familiar with the intertidal
zonation of organisms on those shores. Zonation, in this sense,
means that if you look at the shore from a distance, the life
on it seems to form discrete layers all parallel to sea level.
On the coast of the N. E. Pacific Ocean, the plants and animals
characteristic of the terrestrial environment are not found
down to the water's edge, but their distributions end several
feet above sea level, creating a zone of substrate where neither
terrestrial nor fully marine animals live. This is the intertidal
zone, found between the highest and lowest tides. Typically
the highest intertidal zone is a barren area of little evident
life; it is often an area of bare rock or rock. It gets too
much salt spray for terrestrial animals to tolerate, but not
enough water coverage to keep marine life alive, only a hardy
golden crustose lichen is found there. In areas exposed to
a lot of wave action, the next lower zone is dominated by
small California mussels, Mytilus californianus. Where
the wave action is not as extreme, some different species
of mussels are found. These are in the Mytilus edulis/trossulus/galloprovincialis
complex of species; all of which are virtually identical in
appearance. Just below this will be one or two zones covered
in barnacles, with different barnacle species being the predominant
space-filling organism in each layer. Descending further down
in these intertidal areas, one often finds a zone dominated
by clonal zooxanthellate sea anemones, Anthopleura elegentissma.
Finally, below this are several zones dominated by various
algae.

Rocks in these zones are often found at
steep angles and there are lots of cracks, crevices and channels
distributed throughout the areas. In the bottom of the surge
channels, large green sea anemones, Anthopleura xanthogrammica,
are often found, and along the vertical surfaces are purple
or orange sea stars, the lead actor in this little play, Pisaster
ochraceus. These stars typically are relatively robust
animals, often about 6 inches in diameter, but thick for their
size and very rugged.

About 40 years ago, in a series of interesting
experiments, a researcher from the University of Washington
wanted to determine the effects of sea star predation in this
environment. Over a period of a couple of years, he visited
his study sites on the exposed coast of Washington on every
low tide period and removed all the Pisaster he could
find. Over a couple of years, some extraordinary changes occurred
on the beaches. These normally small, one to three inches
long, mussels increased in abundance tremendously and grew
rapidly, and got far larger than they normally did, reaching
lengths of eight to fourteen inches. They grew over and smothered
the barnacles. Similarly, the kelps that were found only in
the lower areas of the beach started moving upward on the
rocks into shallower water. After about four years, the barnacle
zones had disappeared, and the rocky beach belonged only to
the mussels and kelps.

The Pisaster were selective predators,
and preferred mussels as prey. The lower limit of the previous
mussel zone coincided with the maximum tidal height that the
stars could forage up to without dying from desiccation when
the tide went out. With no stars around, the mussels settled
out of the plankton in the high intertidal and moved downward
where they grew much faster than normal, as they were covered
with water more frequently each day and received a lot more
food. Below them, the barnacles, which had previously been
eaten by the stars when they did not have mussels to eat,
went uneaten and grew much larger than normal. They became
functional attachment sites for kelp sporelings and the kelps
covered them and smothered them. Basically, without the sea
star cleaning out mussels and barnacles, the dominant competitors
for the space in the system took over, and changed the community
significantly. The researcher, Robert T. Paine, referred to
the sea star as a keystone predator, for without it the rocky
intertidal community in that region collapses, much as an
arch collapses without its keystone (Paine, 1966, 1974).

What does this have to do with coral
reef aquaria, you ask? Well, on coral reefs, as in the rocky
intertidal of Washington, much of the observed diversity comes
from sea star predation. Here the sea star that is the most
important predator is the oft-reviled "Crown of Thorns,"
Acanthasterplanci. The higher parts of reef
platforms are dominated by rapidly growing and competitively
dominant coral species, in several genera such as Acropora,
Pocillopora, Seriatopora, and Montipora.
Aquarists, for no good reason, refer to animals in these and
some other genera small-polyped-scleractinians. As we all
are aware, if given enough food and enough light these animals
are rapidly growing and are often capable of overgrowing other
animals and killing them. In the game of biological competition,
second place wins you the silver medal of death awarded by
the grim reaper. With these species, it is not how you play
the game, it is whether you win or die.

Figure 2.Acanthaster planci,
the Crown of Thorns sea star. This individual was about
15 inches in diameter.

The question you
might be asking yourself then is, "Given that these species
are often rapidly growing and competitively dominant corals,
why are there so many different kinds of corals found in these
areas?" The reason for a lot of the coral diversity is
the same reason that may be given for zonation on rocky intertidal
beaches of Northwestern North America; starfish predation
on the competitively dominant animals allows other species
to occupy space. Acanthaster is a major coral predator
in these areas, but it is by no means the only one.

These stars move through the reef, and
they eat corals. But, they don't eat all corals, nor do they
eat continuously. What a star seems to do is eat a coral,
and then wander around a bit and then eat another coral. If
there are a lot of stars eating corals in an area, as there
are during "Crown of Thorns "Plagues,"' the
net result is not a reef flat barren of corals, but an area
polka-dotted with small patches of dead corals.

Such a reef looks awful ..ly
inviting for coral larvae to settle into. These open patches
are often the only places where newly settled coral larvae
can thrive. On a reef, corals are spaced much farther apart
than they are in a reef aquarist's tank. Generally, the distance
between "hand-sized" coral heads is on the order
of a foot or so. If they are any closer together than that,
they fight. They are fighting - competing - for space; and
the loser dies. The reason that they are spaced like that
is that any coral larva that lands within about 6 inches or
so of a moderately sized colony is out competed by the larger
colonies surrounding it and cannot survive.

The open patches left after a crown
of thorns outbreak are open spaces for the next settlement
period of new corals. Coral larvae, called planulae, which
are about the size and shape of the small brown flatworms
visible in many reef tanks, swim through the water. Corals
only make one choice in their life, and that choice, where
to settle and spend the rest of its life, is made by the mature
planula. Corals spawn into the water and the planula is formed
by the cellular division that occurs during the early embryonic
development that occurs after the egg is fertilized. Corals
have relatively big eggs packed with yolk, and the larvae
develop from the egg using the yolk as food. They do not feed,
and after a few days, they look like small flatworms. This
small wormlike creature swims along the bottom, and every
few minutes it will swim to the surface and touch its front
end to the surface. In effect, it "tastes" the surface
for the appropriate chemical flavor; the right flavor indicates
a good place to settle and grow. If there are alot of corals
in the area, these small larvae end up becoming just so much
food for these plankton-feeding animals. But, if there are
some open places where bacteria and maybe the "right"
kind of algae have settled, the surface tastes "right,"
and the little worm sticks itself to the surface. Over the
next few days, it changes into a small single coral polyp
and starts to grow. And the only reason it had room to begin
was due to the coral-eating predator that had passed that
way maybe as much as a couple of years before. After a few
years, the reef is a diverse place with no evidence of the
Acanthaster predation, and then the cycle repeats.
In this way, the coral diversity of a reef is maintained.

So, What is A Sea Star and How Do They
Do It?

Sea stars are categorized
as being in the PHYLUM ECHINODERMATA, a group of about 6,000
species. They are all marine, most are moderate size, relatively
few are tiny and none are truly microscopic. Most of them
live on, or in, the marine benthos or bottom environment.

There are six major living subgroups
within the Echinodermata, called classes:

Additionally, there
are a large number of fossil groups that allow us to know
a lot about the evolution of the Echinoderms. As a group,
the Echinoderms are really odd animals when compared to just
about any other group of animals. Externally, they are distinguished
by the lack of a defined front or back end; rather they are
radially symmetrical, similar in some regards to sea anemones
and corals. This radial symmetry is an evolved or derived
state; however, they start life as a bilateral animal with
a front and a back end, and change into a radial animal through
a relatively drastic metamorphosis. Some species, particularly
among the sea cucumbers, but also among the sea urchins, have
become quite bilaterally symmetrical and have left and right
sides and a back and front. Most echinoderms are radially
symmetrical, and as most of them have appendages that occur
in multiples of five, they are referred to as having pentaradial
symmetry.

They totally lack a head, brain, and large
apparent sensory structures. The nervous system in many of
them is so diffuse that, except for a few very large nerves,
most of the nerves are so small that one needs electron microscopy
to even observe them. The basis for behavior in any echinoderm
is poorly known, at best. With no brain, much of the behavioral
responses seen are assumed to be simple reflexes, however,
many of them have complex behaviors, and how these are mediated
is unknown.

They have an internal skeleton of calcium
carbonate, often with a magnesium component. Except for the
spines of pencil urchins, all of the skeletal structures are
internal; so all the spines in most sea urchins and all other
echinoderms are covered with tissue. One characteristic that
is very important to aquarists, but which is totally invisible
from the outside, is that the bodies of echinoderms are largely
hollow and filed with cavities lined with very thin tissue.

Sea stars are perhaps the classical
Echinoderm, and they are likely the animals most folks think
of when they consider that group. They are categorized as
belonging to the Class Asteroidea, which has about 1500 species.
Sea stars always have a more-or-less flattened, flexible body,
although in some it is pretty stiff. Under each arm or ray
is a groove lined with either two or four rows of tube feet.

Figure 3.Choriaster granulatus,
the Doughboy star, this individual is about 12 inches
across and, contrary to its "puffy" appearance,
it is quite stiff and rigid. This species is also a
predator on corals.

The tube feet are the external manifestations
of an organ system unique to echinoderms, the ambulacral or
water-vascular system. This is a hydraulic system that consists
of thousands of internal pipes, tubes and valves. Each tube
foot is connected by a lateral tube to a radial canal running
down the center of each arm. Fluid is pumped into each tube
foot, and valves may close isolating it. Inside the sea star's
arm, a small balloon-like structure called the ampulla extends
up from the tube foot. The tube foot looks like an eye-dropper
with a flexible tube and no opening. When the eye-dropper
bulb (= ampulla) is closed by muscle contraction, the fluid
in the ampulla is pushed into tube, thereby extending it.
When those ampullar muscles relax and muscles running along
the length of the tube foot contract, the foot is pulled back
in and shortened, and the fluid is moved back into the ampulla
inflating it. The foot can pivot at the point it joins the
body, due to muscles connecting it to the body wall. The tip
of the foot terminates, in most sea stars, in an adhesive
pad, which sticks to the substrate by the use of a temporary
glue. So when a sea star moves, the tube foot is pivoted in
the direction of locomotion, extended, and pivoted back in
a swinging "walking" step. When it contacts the
substrate, a glue fastens it to the substrate (Hermans, 1983).
As the walking step continues the star moves above the foot,
just like you move above your foot when your leg pivots. At
the end of the pivot cycle, the glue is released, and the
tube foot extends off the substrate. It is contracted and
pivoted back to the initial position. Now, this is pretty
easy to visualize for one tube foot, and controlling this
might be pretty easy if you only have a few tube feet, but
some stars have as many as 40,000 tube feet and can move very
rapidly across the substrate! All this is done with no brain
to control or co-ordinate any part of the locomotion, and
how the locomotion is co-ordinated is not known. The neuronal
basis for any behavior is not known for any echinoderm.

Figure 4. A Choriaster,
drawn as if cut open to show the internal structures.
There are two pyloric caeca and two gonads in each arm,
although only one of each is shown for clarity. From
this vantage point, the gut is visible in the center
part of the animal, but the mouth is seen from the insides.
The gut structures are shown in various shades of red,
pink, purple, orange or brown. The gonad is yellow,
and the ambulacral system is in shades of blue.

Internally, they
have a short gut that primarily runs from the central mouth
directly through the animal terminating at the anus, which
is generally in the center of the upper part of the body.
The mouth faces down, and immediately above it is a short
esophagus. The first part of the stomach is located above
this, and it is called the "cardiac" stomach. This
may be extruded in some, but not all, stars to initiate
digestion outside the animal. The cardiac stomach is connected
to the "pyloric" stomach. This "stomach"
is basically the central section of a food storage organ,
and it sends two large sack-like extensions called pyloric
caeca into each arm or ray. These function in food storage
and can vary a lot in size. From the pyloric stomach, a short
intestine extends to the anus. There is a pair of rectal caeca
of uncertain function attached to the rectum.

Above or adjacent to the pyloric caeca
are the gonads which discharge through an external opening
on each side of the ray near the top. There is no sexual dimorphism,
and the sexes are externally identical.

Sea stars feed in a number of different
ways. Many of them, including Pisaster ochraceus and
Acanthaster planci extend their cardiac stomachs outside
the body and digest much of their prey externally. Some of
them, such as the burrowing sand stars, sold erroneously in
the hobby as sand sifters, take their prey internally and
eat it there. One of the largest stars is Pycnopodia helianthoides
found in the Northeastern Pacific, and it eats its prey internally.
In an examination of these stars to see what they were eating,
I have found two of them with the remains of complete diving
ducks inside them, and I have no doubt that they could have
captured the birds, drowned them, and eaten them. Other stars
such as Linckia laevigata, the blue star imported for
the aquarium hobby, and species of Pteraster, eat by
extending their stomach over the substrate, and they basically
try to digest the world. They eat sponges, small microorganisms,
small sessile organisms, and whatever else doesn't walk away.
Finally some stars, such as the blood stars, Henricia
species, found commonly in many temperate seas, extend their
rays up into the water and extend mucus from them. Plankton
adheres to the mucus, and the stars eat both the mucus and
the plankton.

Figure 5.Pycnopodia helianthoides,
the sunflower star. This species reaches diameters of
more than 5 feet and takes its prey internally to digest
it. It is a major predator in many subtidal areas of
the North Eastern Pacific.

More detailed information about sea stars
may be obtained by consulting some of the standard invertebrate
zoology text books (Kozloff, 1990; Ruppert and Barnes, 1994).

Aquarium Concerns

Sea stars are often
delightful animals to watch in the ocean. However, they are
very seldom desirable in our reef aquaria, simply because
they are large and predatory. Many of the larger ones found
on coral reefs are at least potentially predatory on corals,
and having one of these animals in a tank can become an exercise
in watching expensive dining. Other moderately-sized stars
may be kept for a while, but generally will starve to death
in our systems; included in this group are species of Fromia,
most of which appear to require specific sponges and tunicates
as prey.

A few species of stars may be kept successfully
in aquaria. Probably the most ubiquitous of these are several
(?) small species of cushion stars, possibly in the genus
Asterina. These small stars are gray, white, or sometimes
mottled with green, and are about one half inch across. They
reproduce by fission, and are seldom seen with a complete
array of arms. There appear to be three distinct types, which
may be different species, found in reef aquaria. The most
common variety is one that appears to eat algae and surface
films. The second most common variety (although it is quite
rare) eats zoanthids and soft corals. The rarest variety of
these small white stars eats stony corals. Fortunately, aquarium
control of them is pretty easy. They are not the speediest
of animals, and if you find you have a type that is causing
problems, periodic starfish safaris can generally rid a tank
of them.

Larger species of stars have generally
poor success rates in our tanks. Stars such as the chocolate
chip star are simply too predatory to maintain, as are the
so-called sand-sifting stars. Sold to "sift sand,"
these latter species will devastate a live sand bed in short
order. About the only larger stars that are successfully kept
are species of Linckia, both the large Linckia laevigata,
and the smaller Linckia multifora. Both are both surface
deposit feeders and eat algal films, and many small sessile
animals, but tend to leave corals and other ornamental animals
alone.

The major concern for aquarists with any
sea star is, or should be, salinity. These are animals that
do not tolerate changes in salinity at all well, and also
seem to suffer from transport stress. They need full strength
sea water at 35 ppt to 37 ppt salinity. They also do best
at temperatures in the 80º F to 84º F range. They
must be acclimated very slowly; often an acclimation of six
hours or more is called for. Even so, the survival rate of
these beautiful animals is abysmal. Probably less than one
tenth of the ones imported for the hobby survive a week in
a hobbyist's tank. If they survive more than a few days, however,
they will likely do well for a long time.

We depend, in the long run, on sea
stars and their predatory activities for many of the corals
seen in our tanks, and they are often beautiful and interesting
animals. Unfortunately, most of them are NOT suitable
as reef aquarium animals. Even for those few that are suitable,
however, our success rate is pretty poor, and more care needs
to be taken in the shipping, handling, transport, and general
acclimation of them, to ensure their survival in our tanks.

If you have any questions about this article, please visit my author
forum on Reef Central.